The present invention relates to a thermal heat-sensitive transfer material which can give transferred recorded images of good printed letter quality even on a recording medium with poor surface smoothness and a process for production thereof
The thermal or heat-sensitive transfer recording method has advantageous features that it does not require converted ("treated") paper and provides recorded images with excellent durability in addition to the general features of the thermal recording method that the apparatus therefor is light in weight, compact, free of generating noise and also excellent in operability and maintenance. For these reasons the thermal transfer recording method has been recently widely used.
The thermal transfer recording method employs a thermal transfer material, comprising generally a heat transferable ink containing a colorant dispersed in a heat-fusible binder applied on a support generally in the form of a sheet. The thermal transfer material is superposed on the recording medium so that the heat-transferable ink layer may contact the recording medium, and the ink layer, melted by supplying heat by a thermal head from the support side of the thermal transfer material, is transferred onto the recording medium, thereby forming a transferred ink image corresponding to the pattern of the heat supplied on the recording medium.
However, the thermal transfer recording method of the prior art involves some drawbacks. That is, according to the thermal transfer recording method of the prior art, the transfer recording performance, namely printed letter quality is greatly influenced by the surface smoothness, and therefore, although good quality of letter printing can be effected on a recording medium with high smoothness, the printed letter quality will be markedly lowered on a recording medium with poor smoothness. For this reason, a paper having a high surface smoothness is generally used. However, a paper with a high smoothness is rather special and the papers in general possess various degrees of concavities and convexities due to entanglement of fibers. Accordingly, in the case of a paper with a large surface unevenness, the heat-molten ink cannot penetrate into the fibers of the paper during transfer printing, but caused to adhere only at the convexities of the surface or in the vicinity thereof, with the result that the image printed at the edge portion is not sharp or a part of the image may be lacking to lower the printed letter quality. For improvement of the printed letter quality, there has been taken a measure of using a heat-fusible ink having a low melting point at least in the surface layer, or increasing the thickness of the heat-transferable ink layer based on a concept of causing the melted ink to penetrate faithfully into the surface unevenness of paper, etc. When an ink having a low melting point is used, however, the heat transferable ink layer will be sticky at a relatively low temperature to result in lowering in storability or troubles such as staining at non-printed portions of the recording medium or blurring of transferred images. Further, in a case where a transferable ink layer having a large thickness is used, blurring becomes remarkable and a large amount of heat supply from a thermal head is required to lower the printing speed.